Your item bag is available from the very start of the game and is where the player will store all of the items that are obtained throughout the game. For example, pokeballs, insense, potions and other consumable items. Pokemon, Pokemon eggs and any other non items are not stored inside the bag and do not contribute the the total size of the bag. The default bag will be able to hold a maximum of 350 items before it is filled up and can not hold any more items.
Increase Bag Capacity
The capacity of the bag is 350. If you find this is not enough, you will be able to purchase bag upgrades from the in game store. Each tier of the upgrade will increase the total carry capacity of the bag by 50 items.